The present invention relates to a tangential or cross-flow filtration apparatus applicable to the filtration of fine particles suspended in liquids, more particularly in water treatment plants in the agrofood processing and biological manufacturing industries, for example, for the production of extremely pure water, for the disinfection of industrial waste waters, for the sterilization of drinking water, fruit juices or food liquids, for the separation of cellular biomass and, more generally, for any advanced decontamination of liquids and any treatment accomplished on a microporous ultrafiltration or on a microfiltration membrane.
The tangential or cross-flow filtration principle is well known and involves creating a relative displacement at a high velocity of the suspended matter to be filtered and of a porous filtering material in order to avoid or to limit the formation of cakes of the filtered matter on the filtering material. This technique permits filtering of suspended fine particles ranging from 0.1 to a few tens of microns in microfiltration and even of macromolecules with a molecular weight in excess of 10,000, e.g. in ultrafiltration, for which the conventional filtration methods by percolation lead to very low velocities of the filtrate due to very rapid plugging of the filtering material or to the formation of a cake with ever-increasing thickness.
The tangential filtration technique permits a high velocity of filtration and a continuous or semicontinuous operation. Moreover, it permits a perfect definition of the desired degree of filtration in accordance with the geometric characteristics of the filtering material employed.
Several types of filtering elements already have been described and which enable such tangential filtration to be accomplished. The oldest apparatus known, described in Soviet Pat. No. 142,626 to Zhevnovatyi, A. I. in 1961, is formed by a tube of porous material fixed inside a second tube, the suspension to be filtered passing under load at high velocity in the annular space between the two tubes, the filtrate flowing within the porous tube.
Since then, the filtering materials employed have been improved, as well as the flow of the liquid to be filtered. Thus, certain equipment uses two concentric cylinders, with the internal cylinder being formed by a microporous membrane, the liquid being subjected to a forced helicoidal flow around such internal cylinder.
Other devices comprise a series of filtering elements superposed in the form of plates or disks, on the two faces of which microporous membranes are arranged, for example, around a filtrate-collecting tube, the suspension to be filtered passing between the disks in a helicoidal path one after another.
There also are devices with rotating plates with greater mechanical complexity capable of producing corrosion phenomena.
However, the main problem is still the control of the flow of the liquid to be filtered, its distribution on the plates and on the disks, and the limitation due to loss of head of the necessary high speed circulation.
Moreover, the advantageous modular adaptation of the equipment to a particular problem at hand and for ready access to the porous material is usually obtained at the expense of an adequate distribution of the flow.